1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus, and more particularly to a magnetic recording and reproducing apparatus of the type wherein a video signal is FM (frequency modulation) modulated and a digital audio signal (hereinafter referred to as PCM (pulse code modulation) audio signal) is time division multiplexed into a blanking period of the FM video signal to produce a recording signal and then the recording signal is recorded onto and reproduced from a magnetic record medium by means of a rotary magnetic head.
2. Description of the Prior Art
Magnetic recording and reproducing apparatus are already known wherein a PCM audio signal is inserted into a blanking period of an FM video signal to produce a recording signal and the thus produced recording signal is recorded onto and then reproduced from a magnetic record medium by means of a rotary head. An exemplary one of such conventional magnetic recording and reproducing apparatus is shown in FIG. 52. Referring to FIG. 52, the magnetic recording and reproducing apparatus shown is a video tape recorder and includes a recording system including a pre-emphasis circuit 10 for emphasizing high frequency components of a video signal, an FM modulator 11 for FM modulating a video signal, a PCM encoder 12 for processing an input audio signal by shuffling, addition of an error correction code and so forth and time base compressing the thus processed audio signal so as to allow an audio signal for one field period to be inserted into a vertical blanking period of an FM video signal to PCM modulate the audio signal, a pair of changing over switches 13 and 14 for alternatively selecting, upon recording, an FM modulated video signal and a PCM modulated audio signal, an adder 15 for adding an FM modulated video signal and a PCM modulated audio signal outputted from the changing over switches 13 and 14, respectively, a recording amplifier 16 for amplifying an output signal of the adder 15, and a recording video head 17 in the form of a rotary head for recording an output signal of the recording amplifier 16 onto a magnetic record medium in the form of a magnetic tape 200.
The magnetic recording and reproducing system further includes a reproducing system including a reproducing video head 18 in the form of a rotary head, a reproduction amplifier 19 for amplifying an output signal of the reproducing video head 18, a PCM decoder 23 for decoding an output signal of the reproduction amplifier 19, that is, for detecting a PCM audio signal inserted in a vertical blanking period of a reproduction signal and processing the detected PCM audio signal by error correction, de-shuffling and so forth to restore an original audio signal, a reproduction equalizer 20 for compensating for a frequency characteristic of a reproduction FM video signal, an FM demodulator 21 for FM demodulating a reproduction FM video signal, a de-emphasis circuit 22 having a reverse characteristic to that of the pre-emphasis circuit 10, a clamp circuit 24, an analog to digital (A/D) converter 25 for converting a reproduction video signal from an analog signal into a digital signal, a synchronizing separator circuit 26 for separating a synchronizing signal from a reproduction video signal, a reproduction signal processing circuit 27 for processing a reproduction signal by time base correction, dropout compensation and so forth in accordance with a synchronizing separation signal from the synchronizing separator circuit 26, and a digital to analog (D/A) converter 28.
The magnetic recording and reproducing apparatus operates in the following manner. Referring also to FIG. 53 at the top of which a video tape is diagrammatically shown on which a recording signal in which a PCM audio signal is inserted in a vertical blanking period of a video signal is recorded, an video signal inputted, upon recording, to the recording system by way of a video signal input terminal 100 is first supplied to the pre-emphasis circuit 10, at which high frequency components thereof are emphasized, and then to the FM modulator 11, at which it is FM modulated to obtain an FM video signal. Meanwhile, an audio signal inputted to the recording system by way of an audio signal input terminal 101 is supplied to the PCM encoder 12, at which the process of shuffling, addition of an error correction code and so forth is performed therefor and then time base compressing process is performed for the thus processed audio signal in order to allow the audio signal for one field period to be later inserted into a fixed portion of a vertical blanking period of the FM video signal as seen in FIG. 53. The PCM encoder 12 outputs the thus PCM modulated audio signal therefrom. The PCM modulated audio signal from the PCM encoder 12 and the FM modulated video signal from the FM modulator 11 are then alternatively selected by the switches 13 and 14, that is, selectively transmitted to the adder 15 in accordance with a suitable changing over operation of the switches 13 and 14 so that they are added to each other such that the PCM audio signal may be recorded in a recording area which is provided at a fixed portion of a vertical blanking period of an FM modulated video signal as seen in FIG. 54 thereby to produce a recording signal. The recording signal is supplied to the recording amplifier 16, at which the FM video signal and the PCM audio signal thereof are amplified. The composite recording signal of the FM video signal and the PCM audio signal is then recorded onto a magnetic tape 200 by means of the recording video head 17.
On the other hand, upon reproduction, such FM video signal and PCM audio signal recorded in a mixed condition on the magnetic tape 200 are reproduced from the magnetic tape 200 by means of the reproducing video head 18 and amplified as a reproduction signal by the reproduction amplifier 19. The thus amplified reproduction signal is transmitted to the reproduction equalizer 20 and also to the PCM decoder 23. At the PCM decoder 23, a PCM audio signal for one field inserted in a vertical blanking period of such reproduction signal is detected and then processed by time base elongation, error correction, de-shuffling, error modification and so forth to restore an original audio signal. The thus restored original audio signal by the PCM decoder 23 is then outputted as a reproduction output audio signal of the reproducing system by way of a reproduction audio signal output terminal 103.
In the meantime, the reproduction equalizer 20 compensates for a frequency characteristic of the reproduction FM video signal in the reproduction signal received from the reproduction amplifier 19, and an output of the reproduction equalizer 20 is inputted to and FM demodulated by the FM demodulator 21. The FM demodulated video signal is then inputted to the de-emphasis circuit 22, at which high frequency components thereof, which were emphasized upon recording, are de-emphasized to restore an original video signal. The thus restored reproduction video signal is then inputted to the clamp circuit 24, at which it is clamped so that it may have a voltage within a voltage range applicable for the A/D converter 25 at a next stage. It is to be noted that, at the clamp circuit 24, a synchronizing signal of the reproduction video signal is separated to produce a clamp pulse and a leading end portion of the synchronizing signal of the reproduction video signal is clamped to a predetermined dc voltage level in response to such clamp pulse. The thus clamped reproduction video signal from the clamp circuit 24 is then converted from an analog signal to a digital signal by the A/D converter 25 and then transmitted to the reproduction signal processing circuit 27. The clamped reproduction video signal is also transmitted to the synchronizing separator circuit 26, at which a synchronizing signal is separated therefrom. The reproduction signal processing circuit 27 performs reproduction signal processing such as time base correction and dropout compensation of the reproduction video signal from the A/D converter 25 in accordance with a synchronizing separation output of the synchronizing separator circuit 26, and an output reproduction video signal of the reproduction signal processing circuit 27 is converted from a digital signal back into an analog signal by the D/A converter 28 and outputted as a reproduction output video signal of the reproducing system by way of a reproduction video signal output terminal 102.
In the conventional magnetic recording and reproducing apparatus in the form of a video tape recorder described above, an FM modulated video signal and a PCM audio signal are added, upon recording, to each other in accordance with a suitable changing over operation in a fixed portion of a blanking period of the FM video signal. On the other hand, upon reproduction, a reproduction FM video signal and a PCM audio signal, which was mixed with such FM video signal upon recording, are FM demodulated as they are. Consequently, a reproduction video signal including such reproduction FM video signal and PCM audio signal presents such an output signal waveform as shown in FIG. 54 as if it includes noises in a PCM audio signal region.
Further, before processing such as FM demodulation of a reproduction signal in which a PCM audio signal is inserted is performed, an FM video signal and such PCM audio signal are separated from each other. In this instance, a period of a PCM audio signal region of the reproduction FM video signal from which the PCM audio signal has been separated makes a no signal portion, and if the reproduction FM video signal having such no-signal portion is FM demodulated, then the waveform of the FM demodulated video signal presents such an output signal waveform as shown in FIG. 54 as if it includes noises in its PCM audio signal region which is a no-signal portion similarly as described above.
If such a noise signal waveform as seen in FIG. 54 is included in a PCM audio signal region of a reproduction FM video signal, normal synchronizing separation cannot be performed with such reproduction FM video signal. In particular, where the clamp circuit 24, which constitutes together with the A/D converter 25, synchronizing separator circuit 26, reproduction signal processing circuit 27 and D/A converter 28 a time base corrector by which a time base variation of a reproduction video signal caused by elongation of a video tape or the like is time base corrected by detection of an interval of time between adjacent synchronizing signals to re-set a synchronizing signal to a correct position, is constructed such that it separates a synchronizing signal of a reproduction video signal to produce a clamp pulse and clamps an end of a synchronizing signal in accordance with such clamp pulse, if such a noise waveform is included in a PCM audio signal region, correct synchronizing separation cannot be performed with such reproduction video signal, but the clamp circuit 24 will perform a clamping operation at a lower end of a noise portion in the PCM audio signal region. Consequently, the clamp circuit at the input of the analog to digital converter does not operate normally and a time base correcting operation cannot be performed appropriately, which makes a reproduction picture image unstable or abnormal.
Another exemplary conventional magnetic recording and reproducing apparatus is shown in FIG. 55. Referring to FIG. 55, the magnetic recording and reproducing apparatus shown includes a recording system including an analog to digital (A/D) converter 1 for converting an input video signal from an analog signal into a digital signal, a first synchronizing separator circuit 2 for separating a synchronizing signal from an input video signal, a recording signal processing circuit 3 for performing such recording signal processing as to insert blanking data, which make, upon reproduction, a reference signal for such processing as time base correction, into a video signal in accordance with a synchronizing signal from the first synchronizing separator 2, a vertical blanking data generator 4 for generating, in accordance with a synchronizing separation signal from the first synchronizing separator circuit 2, blanking data for a vertical blanking period of a video signal, a gate pulse for enabling vertical blanking data to be inserted into video signal data from the recording signal processing circuit 3 and a timing pulse for inserting a PCM audio signal into an FM video signal, a selector 5 for inserting vertical blanking data from the vertical blanking data generator 4 into video signal data from the recording signal processing circuit 3, and a digital to analog (D/A) converter 6 for converting an output signal of the selector 5 from a digital signal into an analog signal. The recording system of the magnetic recording and reproducing apparatus further includes an FM modulator 11, a PCM encoder 12, a pair of switches 13 and 14, an adder 15, a recording amplifier 16 and a recording video head 17, which are similar to the FM modulator 11, PCM encoder 12, switches 13 and 14, adder 15, recording amplifier 16 and recording video head 17, respectively, of the recording system of the magnetic recording and reproducing apparatus shown in FIG. 52. It is to be noted that the pre-emphasis circuit 10 of the magnetic recording and reproducing apparatus of FIG. 52 is omitted from the magnetic recording and reproducing apparatus of FIG. 55 and an output digital signal of the D/A converter 6 is inputted to the FM modulator 11.
The magnetic recording and reproducing apparatus further includes a reproducing system which is similar in construction to but only different from the reproducing system of the magnetic recording and reproducing apparatus shown in FIG. 52 in that it does not include the de-emphasis circuit 22. It is to be noted that, since the recording system includes the first synchronizing separator circuit 2, the synchronizing separator circuit 26 may be hereinafter referred to as second synchronizing separator circuit.
In operation, an input video signal is first converted from an analog signal into a digital signal by the A/D converter 1 and then outputted to the recording signal processing circuit 3. Meanwhile, the input video signal is inputted also to the first synchronizing separator circuit 2, at which a synchronizing signal is separated from the input video signal. At the recording signal processing circuit 3, such recording signal processing as to insert blanking data, which make, upon reproduction, a reference signal for such processing as time base correction, into a video signal is performed in accordance with an output signal from the first synchronizing separator 2, and the thus processed signal is transmitted from the recording signal processing circuit 3 to the selector 5. Meanwhile, at the vertical blanking data generator 4, vertical blanking data for a video signal are generated with reference to the output of the first synchronizing separator circuit 2 and outputted to the selector 5. Here, the vertical blanking data generator 4 further operates to generate a gate pulse for enabling vertical blanking data to be inserted into video signal data outputted from the recording signal processing circuit 3 and a timing pulse for inserting a PCM audio signal into an FM video signal.
At the selector 5, the vertical blanking data received from the vertical blanking data generator 4 are inserted into the video signal data received from the recording signal processing circuit 3 with reference to a gate pulse from the vertical blanking data generator 4. Then, an output of the selector 5 is converted from a digital signal into an analog signal by the D/A converter 6. A typical waveform of such recording video signal outputted from the D/A converter 6 is shown in FIG. 56. The recording video signal outputted from the D/A converter 6 is subsequently supplied to the FM modulator 11 while an input audio signal is supplied to the PCM encoder 12, whereafter the video signal and audio signal are processed by the FM modulator 11 and PCM encoder 12, switches 13 and 14, adder 15 and recording amplifier 16 and recorded onto a magnetic tape 200 by means of the recording video head 17 in a similar manner as in the magnetic recording and reproducing apparatus of FIG. 52 described hereinabove. It is to be noted here that changing over operations of the switches 13 and 14 are performed in response to a timing pulse transmitted thereto from the vertical blanking data generator 4. Also the PCM encoder 12 operates in response to such timing pulse from the vertical blanking data generator 4.
The thus recorded video signal and audio signal are reproduced from the magnetic tape 200 by means of the reproducing video head 18 and processed in a similar manner as in the reproducing system of the magnetic recording and reproducing apparatus of FIG. 52 except that an FM demodulated reproduction video signal from the FM demodulator 21 is supplied directly to the clamp circuit 24 without being processed by any de-emphasis circuit because pre-emphasis has not been performed for a recording video signal in the recording system.
Also in the conventional magnetic recording and reproducing apparatus of FIG. 55, an FM modulated video signal and a PCM audio signal are added, upon recording, to each other in accordance with a suitable changing over operation such that the PCM audio signal is inserted into a fixed portion of a blanking period of the FM video signal. On the other hand, upon reproduction, a reproduction FM video signal and a reproduction PCM audio signal, which was mixed with such FM video signal upon recording, are FM demodulated as they are. On the other hand, since a PCM audio signal has a waveform of pulses, it includes wide band signal components, and consequently, it is not certain what waveform a video signal after demodulation has in a PCM audio signal region as seen from a waveform shown in FIG. 54. Rather, it has such a signal waveform as if it includes noises in a PCM audio signal region.
Further, also in such a case that an FM video signal and such PCM audio signal are separated from each other as seen from a waveform (b) of FIG. 57 before processing such as FM demodulation of a reproduction signal in which a PCM audio signal is inserted as seen from another waveform (a) of FIG. 57 is performed, a period of a PCM audio signal region of the reproduction FM video signal (waveform (b) of FIG. 57) from which the PCM audio signal has been removed makes a no-signal portion, and if the reproduction FM video signal having such no-signal portion is FM demodulated, then the waveform of the FM demodulated video signal presents such an output signal waveform (c) as shown in FIG. 57 as if it includes noises in its PCM audio signal region which is the no-signal portion, similarly as described.
If such a noise signal waveform (c) as seen in FIG. 57 is included in a PCM audio signal region of a reproduction FM video signal, normal synchronizing separation cannot be performed with such reproduction FM video signal. In particular, where the clamp circuit 24 at a preceding stage to the A/D converter 25 is constructed such that it separates a synchronizing signal of a reproduction video signal to produce a clamp pulse and clamps an end of a synchronizing signal in accordance with such clamp pulse, if such a noise waveform (c) as seen in FIG. 57 is included in a PCM audio signal region, correct synchronizing separation cannot be performed with such reproduction video signal, but the clamp circuit 24 will perform a clamping operation at a lower end of a noise portion in the PCM audio signal region. Consequently, the clamp circuit 24 at the input of the analog to digital converter 25 will not operate normally and a time base correcting operation cannot be performed appropriately, which makes a reproduction picture image unstable or abnormal.
Further, it is not certain what waveform a video signal has, after demodulation, in a PCM audio signal region of a reproduction video signal, and if a noise is outputted at a higher level than a white level of a video signal or at a level lower than an end of a synchronizing signal as seen in FIG. 54 or else a dc component is included, such noise will cause a sag, by which the reproduction video signal is partially varied in hue. Consequently, a good reproduction video signal cannot be obtained.
A further exemplary conventional magnetic recording and reproducing apparatus is shown in FIG. 59. Referring to FIG. 59, the magnetic recording and reproducing apparatus shown includes a recording system which is similar in construction to the recording system of the magnetic recording and reproducing apparatus shown in FIG. 55 except that it includes a recording signal processing circuit 7 in place of the recording signal processing circuit 3, vertical blanking data generator 4 and selector 5 of the magnetic recording and reproducing apparatus of FIG. 55 and additionally includes a pre-emphasis circuit 10 connected to the recording signal processing circuit 7. The recording signal processing circuit 7 performs recording signal processing such as to insert blanking data into an input video signal with reference to a synchronizing signal outputted from the first synchronizing separator circuit 2. The recording signal processing circuit 7 further operates to generate a timing pulse for inserting a PCM audio signal into an FM video signal. Such timing signal is transmitted to the PCM encoder 12 and switches 13 and 14. An input video signal processed by the recording signal processing circuit 7 is inputted to the D/A converter 6, at which it is converted from a digital signal into an analog signal. An analog output video signal of the D/A converter 6 is inputted to the pre-emphasis circuit 10, at which high frequency components thereof are emphasized, and then inputted to the FM modulator 11. It is to be noted that the magnetic recording and reproducing apparatus includes a pair of rotary heads 29a and 29b which each acts as a recording head upon recording and as a reproducing head upon reproduction.
The magnetic recording and reproducing apparatus further includes a reproducing system which is also similar in construction to the reproducing system of the magnetic recording and reproducing apparatus of FIG. 55 in that it includes a pair of reproduction amplifier 19a and reproduction amplifier 19b and a PCM decoder 31 in place of the single reproduction amplifier 19 and the PCM decoder 23, respectively, and additionally includes a head changing over circuit 30 interposed between the reproduction amplifiers 19a and 19b and head changing over circuit 30, a limiter 32 interposed between the reproduction equalizer 20 and FM demodulator 21 and a de-emphasis circuit 22 interposed between the FM demodulator 21 and clamp circuit 24. The reproduction amplifiers 19a and 19b amplify reproduction signals from the rotary heads 29a and 29b, respectively, and the thus amplified reproduction signals are selectively received by the head changing over circuit 30 in response to a head changing over signal. The PCM decoder 31 may be similar in construction to the PCM decoder 23 described hereinabove. The limiter 32 receives an output reproduction FM video signal of the reproduction equalizer 20 and removes amplitude variation components from the received reproduction FM video signal. The de-emphasis circuit 22 has a reverse characteristic to that of the pre-emphasis circuit 10 of the recording system.
Referring to FIG. 60, the rotary heads 29a and 29b are shown as viewed from above. The rotary heads 29a and 29b are mounted on a rotary drum 44 in an angularly spaced relationship by 180 degrees from each other. A magnetic tape 200 is wrapped over about 180 degrees around an outer periphery of the rotary drum 44 under the guidance of a pair of guide pins 45a and 45b and fed in a direction indicated by an arrow mark by the rotary drum 44 when the rotary drum 44 rotates in a counterclockwise direction in FIG. 60.
In operation, an input video signal is first converted from an analog signal into a digital signal by the A/D converter 1 and then outputted to the recording signal processing circuit 3. At the recording signal processing circuit 3, such recording signal processing as to insert blanking data is performed in accordance with a synchronizing separation output of the first synchronizing separator 2. The recording signal processing circuit 3 also generates and outputs a timing pulse for inserting a PCM audio signal into an FM video signal. An output recording signal of the recording signal processing circuit 3 is converted from a digital signal into an analog signal by the D/A converter 6 and then inputted to the pre-emphasis circuit 10, at which high frequency components thereof are emphasized. The recording video signal is subsequently inputted to the FM modulator 11 while an input audio signal is supplied to the PCM encoder 12, whereafter the video signal and audio signal are processed by the FM modulator 11 and PCM encoder 12, switches 13 and 14, adder 15 and recording amplifier 16 in a similar manner as in the magnetic recording and reproducing apparatus of FIGS. 52 and 55 described hereinabove and then recorded onto a magnetic tape 200 by means of the rotary heads 29a and 29b. It is to be noted here that changing over operations of the switches 13 and 14 are performed in response to a timing pulse transmitted thereto from the recording signal processing circuit 7. Also the PCM encoder 12 operates in response to such timing pulse from the vertical blanking data generator 4.
In this instance, since the magnetic tape 200 is wrapped over a little greater than 180 degrees as seen in FIG. 60 on the outer periphery of the rotary drum 44, even if recording electric current flows always through the rotary heads 29a and 29b, recording on the magnetic tape 200 takes place only while a rotary head is in contact with the magnetic tape 200 within the angular range. Accordingly, the magnetic tape has such record pattern as illustrated in FIG. 61 wherein information is recorded in two regions a and b while the rotary drum 44 makes one full rotation such that a recording signal for one field is recorded onto one track. It is to be noted that, in FIG. 61, reference character a denotes a track recorded by the rotary head 29a while reference character b denotes a track recorded by the other rotary head 29b. Further, since a PCM audio signal is recorded in a time division multiplexed condition in a vertical blanking period of an FM video signal, such PCM audio signal is recorded at each of hatched portions of the tracks a and b.
The thus recorded FM video signal and PCM audio signal are reproduced from the magnetic tape 200 by means of the rotary heads 29a and 29b and then amplified by the reproducing amplifiers 19a and 19b. Then, the reproduction signals outputted from the reproduction amplifiers 19a and 19b are alternatively selected by the head changing over circuit 30 in response to a head changing over signal such that a reproduction signal of a rotary head which is currently in contact with the magnetic head 200 may be selected to form such a single reproduction signal as shown by a waveform (c) of FIG. 62 as seen from waveforms (a) and (b) of FIG. 62. The reproduction signal from the head changing over circuit 30 is then transmitted to the reproduction equalizer 20 and also to the PCM decoder 31. At the PCM decoder 31, a PCM audio signal for one field inserted in a vertical blanking period of the received reproduction signal is detected and then such signal processing as error correction, de-shuffling, error modification and so forth is performed for the thus detected PCM audio signal to restore an original audio signal. The thus restored audio signal is outputted as an output signal of the recording system from the PCM decoder 31.
Generally, when an FM signal is recorded or reproduced by means of an electromagnetic converting system employing a tape and a head, lower side band waves are emphasized while upper side band waves are suppressed, and consequently, a modulation index is varied and an imbalance of side band waves is produced simultaneously. As a result, a reproduction FM signal which corresponds to a rising portion (waveform (a) in FIG. 63) of a video signal having high frequency components emphasized by the pre-emphasis circuit 10 is distorted significantly as seen from a waveform (b) of FIG. 63. Thus, at the reproduction equalizer 20, lower side band waves are suppressed while upper side band waves are emphasized in such a frequency characteristic as, for example, shown in FIG. 64 thereby to compensate for a frequency characteristic of the reproduction FM video signal. As a result, an output of the reproduction equalizer 20 is reduced in distortion as seen from a waveform (c) of FIG. 63. The output of the reproduction equalizer 20 is transmitted to the limiter 32, at which amplitude variation components of the reproduction FM video signal are removed. The limiter 32 thus develops such an output as shown by a waveform (d) in FIG. 63.
The reproduction signal outputted from the limiter 32 is inputted to and FM demodulated by the FM demodulator 21, and then high frequency components which were emphasized upon recording are de-emphasized by the de-emphasis circuit 22 to obtain a reproduction video signal which are reduced in high frequency noises. The thus obtained reproduction video signal is then inputted to the clamp circuit 24 and thereafter processed by the clamp circuit 24, A/D converter 25, second synchronizing separator circuit 26, reproduction signal processing circuit 27 and D/A converter 28 to produce an output analog reproduction video signal in a similar manner as in the recording system of the magnetic recording and reproducing apparatus of FIG. 52 described hereinabove.
Also in the conventional magnetic recording and reproducing apparatus, a PCM audio signal is time division multiplexed, upon recording, into a vertical blanking period of an FM modulated video signal, and upon reproduction, such reproduction FM video signal and PCM audio signal mixed with such FM video signal are FM demodulated as they are. On the other hand, since a PCM audio signal is a waveform of pulses, it includes wide band signal components, and consequently, a PCM audio signal band will overlap with a video signal band as seen from FIG. 65. In such case, if a reproduced FM video signal and a PCM audio signal are FM demodulated as described above, it is not certain in what waveform an output of the FM demodulator 21 is demodulated in a PCM audio signal region. Rather, the output of the FM demodulator 21 presents such an output signal waveform as shown in FIG. 54 as if it includes noises in a PCM audio signal region.
Further, also in such a case that an FM video signal and such PCM audio signal are separated from each other as seen from the waveform (b) of FIG. 57 before processing such as FM demodulation of a reproduction signal shown by the waveform (a) of FIG. 57 is performed, a period of the PCM audio signal region of the reproduction FM video signal (waveform (b) of FIG. 57) makes a no-signal portion, and if the reproduction FM video signal having such no-signal portion is FM demodulated, the waveform of the FM demodulated video signal presents such output signal waveform (c) as shown in FIG. 57 as if it includes noises in its PCM audio signal region which is the non signal portion similarly as described above.
Where the clamp circuit 24 at a preceding stage to the A/D converter 25 is constructed such that it separates a synchronizing signal of a reproduction video signal to produce a clamp pulse and clamps an end of a synchronizing signal in accordance with such clamp pulse, if such a noise waveform (c) as seen in FIG. 57 is included in a PCM audio signal region of a reproduction video signal, correct synchronizing separation cannot be performed with such reproduction video signal, but the clamp circuit 24 will perform a clamping operation at a lower end of a noise portion in the PCM audio signal region. Consequently, the signal level of an input reproduction video signal to the A/D converter 25 cannot be clamped at a normal value, which makes a reproduction picture image unstable or abnormal.
Further, if it is not uncertain in what waveform an output of the FM demodulator 21 is demodulated in a PCM audio signal region of a reproduction video signal and the output of the FM demodulator 21 has noises above a white level or below an end of a synchronizing signal as seen in FIG. 54 or has a dc component, such noise may cause a sag, by which the reproduction video signal will partially be varied in hue. Consequently, a good reproduction video signal cannot be obtained.